1. Field of the Invention
This invention relates to the integral .beta.-quenched surface regions formed in situ on bulk structures of zirconium alloys by laser beam scanning.
2. Description of Prior Art
Zirconium alloys are now widely accepted as cladding and structural materials in water-cooled, moderated boiling water and pressurized water nuclear reactors. These alloys combine a low neutron absorption cross section with good corrosion resistance and adequate mechanical properties.
The most common zirconium alloys used up to now are Zircaloy-2 and Zircaloy-4. The nominal compositions of these alloys are given in Table 1.
TABLE I ______________________________________ Zircaloy-2 Element Weight ______________________________________ Sn 1.2-1.7 Fe 0.07-0.20 Cr 0.05-0.15 Ni 0.03-0.08 Zr Balance ______________________________________ Zircaloy-4 Element Weight % Sn 1.2-1.7 Fe 0.18-0.24 Cr 0.07-0.13 Zr Balance ______________________________________
In addition to Zircaloy-2 and Zircaloy-4, considerable amount of experimental work and some nuclear work has been done on Zr+15% Nb+0-1% X alloys where X is usually a transition metal.
In general, these materials have proved adequate under nuclear reactor operating conditions. The fuel element design engineer would like a cladding material that is more resistant to high temperature aqueous corrosion while maintaining an adequate mechanical strength.
During manufacture of Zircaloy channels, a seam in the channels is welded together. It has been observed that this seam weld is substantially more resistant to accelerated nodular corrosion than the rest of the unwelded channel. In addition, other work in the literature has shown that accelerated nodular corrosion in a high temperature, high pressure steam environment can be inhibited by .beta.-phase heat treatments which are similar to the effect derived when the welded seams cool down through the .beta.-phase region immediately after welding.
The exact reason for the enhanced resistance of .beta.-quenched Zircaloy to accelerated nodular corrosion in a high temperature, high pressure steam environment is not understood completely. It appears, however, that this enhanced corrosion resistance is related to the fine equiaxed grain structure and to the fine dispersion of iron, nickel and chromium intermetallics in .beta.-quenched Zircaloy. The effect of .beta.-quenching on the metallurgical structure of Zircaloy stems from the fact that .beta. is the high temperature phase of Zircaloy that is not stable below 810.degree. C. and the fact that iron, nickel and chromium are .beta.-stabilizers that partition preferentially to the .beta. phase.
Referring now to FIG. 1, if a Zircaloy sample is held in the .alpha.+.beta. phase region that ranges between 810.degree. C. to 970.degree. C., the Zircaloy transforms to a two phase mixture of .alpha. and .beta. grains. Iron, nickel and chrome being .beta. -stabilizers will segregate to the .beta. phase grains. On cooling the Zircaloy sample from this two phase region through the boundary between the .alpha.+.beta. and .alpha. regions into the .alpha. region, the .beta. phase decomposes precipitating fine grains of .alpha.-zirconium and rejecting the iron, nickel and chrome intermetallics on the adjacent grain boundaries of the newly formed .alpha. grains. The resulting metallurgical structure of the Zircaloy is thus a fine grained .alpha. structure with a fine dispersion of iron, nickel and chromium intermetallics distributed therein. A similar metallurgical structure can be achieved by quenching directly from the .beta.-phase region about 970.degree. C. This heat treatment results in a very fine grain .alpha. "basket weave" structure with a fine distributionof iron, nickel and chromium intermetallics dispersed therein. This latter heat treatment parallels the thermal history of a weld on cooling and results in a metallurgical structure with enhanced resistance to accelerated nodular corrosion in high pressure, high temperature steam. Not only do the Zircaloys, but also Zr+15%Nb+0-1%X (where X is usually a transition metal) alloys exhibit enhanced corrosion resistance in the .beta.-quenched condition. Such a .beta.-quench or .alpha.+.beta. quench is not always feasible for bulk Zircaloy pieces because forming operations, mechanical property requirements, and the generation of large thermal stresses or large thermal distortions in a bulk Zircaloy body may prevent such a quenching operation. In such cases, other ways must be found to prevent the accelerated nodular corrosion of Zircaloy that occurs in steam at high pressures and temperatures.
Accelerated corrosion of Zircaloy-2 and Zircaloy-4 has been observed under boiling water nuclear reactor conditions and appears to initiate at localized spots and spreads across the Zircaloy surface by lateral growth such that in the initial stages of growth these thick light-colored oxide nodules appear like islands on a thin homogeneous dark oxide background. This accelerated corrosion process that occurs in high-temperature, high-pressure steam can be inhibited metallurgically by quenching Zircaloy from its high temperature body centered cubic .beta. form. .beta.-quenched Zircaloy tends to form a thin coherent protective oxide in a high temperature (500.degree. C.) and a high pressure (100 atm) steam environment, that is substantially more resistant to the in-reactor corrosion than Zircaloy that has not received a .beta.-phase heat treatment.
Unfortunately, a .beta.-phase heat treatment reduces the mechanical strength of Zircaloy and markedly increases the strain rate at which strain rate sensitivities indicative of superplasticity are observed. This high strain rate sensitivity and lower strength is caused by grain boundary sliding on a greatly increased grain boundary area due to a finer grain size in .beta.-quenched Zircaloy. Because of these mechanical deficiencies, bulk .beta.-quenched Zircaloy is not particularly desirable for use as cladding and structural materials for water-cooled nuclear reactors.
Despite the potential detrimental effect of .beta.-quenching on the mechanical properties of Zircaloy, bulk .beta.-quenching of Zircaloy channels for nuclear reactors has been commercialized because of the superior corrosion resistance of .beta.-quenched Zircaloy. This commercial process consists of passing a Zircaloy channel through an induction heater to heat the channel into the two phase .alpha.+.beta. region. The channel is subsequently rapidly quenched by spraying water on the hot channel. Although this induction-heating-water-spray process imparts the desired corrosion resistant properties to the Zircaloy channel, it suffers from several deficiencies.
First, the exposure of the Zircaloy channel to oxygen and water during the induction heating and water quenching allows a thick black oxide to form on the channel that subsequently must be removed. This removal step adds to the manufacturing cost of the channel.
Secondly, although it is only necessary to heat treat the surface layers of the channel, the current commercial process exposes the entire channel bulk to the heat treatment required only by the surface layers. The resulting change in mechanical properties of the channel under long term creep conditions may not be desirable.
It is therefore desirable to have a new type of .beta.-quenched Zircaloy that can be used in circumstances where bulk .beta.-quenched Zircaloy can not be used because of its deficient mechanical properties, because of the formation of black scale on its surface and/or because of thermal distortions and thermal stresses such bulk quenching would generate.
An object of this invention is to provide a new and improved zirconium alloy with an integral .beta.-quenched surface region, the composite structure of which overcomes the deficiencies of the prior art.
Another object of this invention is to provide a new form of a zirconium alloy that can be utilized in circumstances where a bulk .beta.-quenched zirconium alloy cannot be used.
Another object of this invention is to provide an integral protective, corrosion-resistant surface region on a zirconium alloy body.
Another object of this invention is to provide a body of zirconium alloy with an integral surface region of .beta.-quenched material formed in situ by heating and rapidly self-quenching the material of the surface region.
Other objects of this invention will, in part, be obvious and will, in part, appear hereafter.